Add __attribute__((__retain__)) to LLVM_ATTRIBUTE_USED

Without the retain attribute the dump functions will be stripped when
LLVM is compiled with `-ffunction-section -Wl,--gc-sections` on
ELF-based systems.

Author: MatzeB

llvm/include/llvm/Support/Compiler.h

@@ -224,7 +224,9 @@
 #define LLVM_PREFETCH(addr, rw, locality)
 #endif
 
-#if __has_attribute(used)
+#if __has_attribute(used) && __has_attribute(retain)
+#define LLVM_ATTRIBUTE_USED __attribute__((__used__, __retain__))
+#elif __has_attribute(used)
 #define LLVM_ATTRIBUTE_USED __attribute__((__used__))
 #else
 #define LLVM_ATTRIBUTE_USED